This invention relates to the purification of natural gas, and, more particularly, to the removal of nitrogen and/or carbon dioxide and recovery of C3+ hydrocarbon from natural gas by use of a novel pressure swing adsorption (PSA) process.
The removal of nitrogen and acid gases such as carbon dioxide from natural gas is of considerable importance inasmuch as nitrogen and carbon dioxide can be present to a significant extent. Nitrogen and carbon dioxide contamination lower the heating value of the natural gas and increase the transportation cost based on unit heating value.
Applications aimed at removing nitrogen, carbon dioxide, and other impurities from natural gas steams streams provide significant benefits to the U.S. economy. In 1993, the Gas Research Institute (GRI) estimated that about one third of the natural gas reserves in the U.S. are defined as sub-quality due to contamination with nitrogen, carbon dioxide, and sulfur. Many of these reserves, however, have discounted market potential, if they are marketable at all, due to the inability to cost effectively remove the nitrogen and carbon dioxide. Nitrogen and carbon dioxide are inert gases with no BTU value and must be removed to low levels (4% total inerts typically and 2% carbon dioxide) before the gas can be sold.
Concurrently, the U.S. has proven reserves of natural gas totaling 167 trillion cubic feet. Over the past five years, annual consumption has exceeded the amount of new reserves that were found. This trend could result in higher cost natural gas and possible supply shortages in the future. As the U.S. reserves are produced and depleted, finding new, clean gas reserves involves more costly exploration efforts. This usually involves off shore exploration and/or deeper drilling onshore, both of which are expensive. Moreover, unlike crude oil, it is expensive to bring imports of natural gas into North America, therefore pricing of natural gas could be expected to rise forcing end users to seek alternative fuels, such as oil and coal, that are not as clean burning as gas. While base consumption for natural gas in the U.S. is projected to grow at 2-3% annually for the next ten years, one segment may grow much more rapidly. Natural gas usage in electric power generation is expected to grow rapidly because natural gas is efficient and cleaner burning allowing utilities to reduce emissions. Accordingly, there is a need to develop a cost-effective method of upgrading currently unmarketable sub-quality reserves in the U.S. thereby increasing the proven reserve inventory.
Methods heretofore known for purification of natural gas, in particular, nitrogen removal, may be divided roughly into three classifications:
(a) Methods involving fractional distillation at low temperature and (usually) high pressure, i.e. cryogenics. Since nitrogen has a lower boiling point than methane and the other hydrocarbons present in natural gas, it may be removed as a gas on liquefying the remaining constituents which are then revaporized.
(b) By selective adsorption of the methane and higher hydrocarbons on an adsorbent such as activated carbon. The adsorbed gases are then desorbed to give a gas free of nitrogen.
(c) Miscellaneous processes involving selective diffusion through a series of organic membranes, formation of lithium nitride by treatment with lithium amalgam, absorption of the nitrogen in liquid ammonia or in liquid sulfur dioxide.
The principal disadvantage of the fractional distillation and adsorption processes is that they remove the major component, methane, from the minor component, nitrogen, instead of the reverse. In cryogenic processing, almost the entire volume of natural gas must be refrigerated, usually compressed, and then heated again. Accordingly, cryogenic processing is expensive to install and operate, limiting its application to a small segment of reserves. Cryogenic technology is believed only capable of cost effectively purifying reserves, which exceed 50,000,000 standard cubic feet of gas per day. Gas reserves that do not fit these criteria are not currently being purified. The potential value of this gas.is totally lost as the wells are usually capped. The processes suggested under paragraph (c) above are handicapped by an unsatisfactory degree of separation or by the use of very expensive materials.
In smaller-scale natural gas operations as well as in other areas such as synthesis gas and coke oven gas processing, adsorption processes can be especially well suited. The adsorption capacities of adsorption units can, in many cases, be readily adapted to process gas mixtures of varying nitrogen content without equipment modifications, i.e. by adjusting adsorption cycle times. Moreover, adsorption units can be conveniently skid-mounted, thus providing easy mobility between gas processing locations. Further, adsorption processes are often desirable because more than one component can be removed from the gas. As noted above, nitrogen-containing gases often contain other gases that contain molecules having smaller molecular dimensions than nitrogen, e.g., for natural gas, carbon dioxide, oxygen and water.
U.S. Pat. No. 2,843,219 discloses a process for removing nitrogen from natural gas utilizing zeolites broadly and contains specific examples for the use of zeolite 4A. The process disclosed in the patent suggests use of a first nitrogen selective adsorbent zeolite in combination with a second methane selective adsorbent. The molecular sieve adsorbent for removing nitrogen is primarily regenerated during desorption by thermal swing. A moving bed adsorption/desorption process is necessary for providing sufficient heat for the thermal swing desorption. The moving bed process specifically disclosed in this patent is not practical and it does not provide a cost efficient method for the separation of nitrogen from natural gas in view of high equipment and maintenance costs and degradation of the adsorbent by attrition due to contact with the moving adsorbent particles.
Despite the advantageous aspects of adsorption processes, the adsorptive separation of nitrogen from methane has been found to be particularly difficult. The primary problem is in finding an adsorbent that has sufficient selectivity for nitrogen over methane in order to provide a commercially viable process. In general, selectivity is related to polarizability, and methane is more polarizable than nitrogen. Therefore, the equilibrium adsorption selectivity of essentially all known adsorbents such as large pore zeolites, carbon, silica gel, alumina, etc. all favor methane over nitrogen. However, since nitrogen is a smaller molecule than methane, it is possible to have a small pore zeolite which adsorbs nitrogen faster than methane. Clinoptilolite is one of the zeolites which has been disclosed in literature as a rate selective adsorbent for the separation of nitrogen and methane.
U.S. Pat. No. 4,964,889 discloses the use of natural zeolites such as clinoptilolites having a magnesium cation content of at least 5 equivalent percent of the ion-exchangeable cations in the clinoptilolite molecular sieve for the removal of nitrogen from natural gas. The patent discloses that the separation can be performed by any known adsorption cycle such as pressure swing, thermal swing, displacement purge or nonadsorbable purge, although pressure swing adsorption is preferred. However, this patent is silent as to specifics of the process, such as steps for treating the waste gas, nor is there disclosure of a high overall system recovery.
It is well-known to remove acid gases such as hydrogen sulfide and carbon dioxide from natural gas streams using an amine system wherein the acid gases are scrubbed from the feed with an aqueous amine solvent with the solvent subsequently stripped of the carbon dioxide or other acid gases with steam. These systems are widely used in industry with over 600 large units positioned in natural gas service in the U.S. The amine solvent suppliers compete vigorously and the amines used range from DEA to specialty formulations allowing smaller equipment and operating costs while incurring a higher solvent cost. These systems are well accepted although they are not very easy to operate. Keeping the amine solvents clean can be an issue.
Another disadvantage to using aqueous amines is that the natural gas product of an aqueous amine system is water saturated . Accordingly, dehydration typically using glycol absorption would be required on the product stream after the carbon dioxide has been removed adding operational and capital costs to the purification process.
For smaller volume applications where gas flows are less than five to ten million cubic feet per day, considerable attention has been given to the development of pressure swing adsorption (PSA) processes for removal of gaseous impurities such as CO2.
Numerous patents describe PSA processes for separating carbon dioxide from methane or other gases. One of the earlier patents in this area is U.S. Pat. No. 3,751,878, which describes a PSA system using a zeolite molecular sieve that selectively adsorbs CO2 from a low quality natural gas stream operating at a pressure of 1000 psia, and a temperature of 300xc2x0 F. The system uses carbon dioxide as a purge to remove some adsorbed methane from the zeolite and to purge methane from the void space in the column. U.S. Pat. No. 4,077,779, describes the use of a carbon molecular sieve that adsorbs CO2 selectively over hydrogen or methane. After the adsorption step, a high pressure purge with CO2 is followed by pressure reduction and desorption of CO2 followed by a rinse at an intermediate pressure with an extraneous gas such as air. The column is then subjected to vacuum to remove the extraneous gas and any remaining CO2.
U.S. Pat. No. 4,770,676, describes a process combining a temperature swing adsorption (TSA) process with a PSA process for the recovery of methane from landfill gas. The TSA process removes water and minor impurities from the gas, which then goes to the PSA system, which is similar to that described in U.S. Pat. No. 4,077,779 above, except the external rinse step has been eliminated. CO2 from the PSA section is heated and used to regenerate the TSA section. U.S. Pat. No. 4,857,083, claims an improvement over U.S. Pat. No. 4,077,779 by eliminating the external rinse step and using an internal rinse of secondary product gas (CO2) during blowdown, and adding a vacuum for regeneration. The preferred type of adsorbent is activated carbon, but can be a zeolite such as 5A, molecular sieve carbons, silica gel, activated alumina or other adsorbents selective of carbon dioxide and gaseous hydrocarbons other than methane.
U.S. Pat. No. 4,915,711, describes a PSA process that uses adsorbents from essentially the same list as above, and produces two high purity products by flushing the product (methane) from the column with the secondary product (carbon dioxide) at low pressure, and regenerating the adsorbent using a vacuum of approximately 1 to 4 psia. The process includes an optional step of pressure equalization between columns during blowdown. U.S. Pat. No. 5,026,406 is a continuation-in-part of U.S. Pat. No. 4,915,711 with minor modifications of the process.
U.S. Pat. No. 5,938,819 discloses removing CO2 from landfill gas, coal bed methane and coal mine gob gas, sewage gas or low quality natural gas in a modified PSA process using a clinoptilolite adsorbent. The adsorbent has such a strong attraction to CO2 that little desorption occurs even at very low pressure. There is such an extreme hysteresis between the adsorption of the adsorbent and desorption isotherms, regeneration of the adsorbent is achieved by exposure to a stream of dry air.
In general, first applications of PSA processes were performed to achieve the objective of removing smaller quantities of adsorbable components from essentially non-adsorbable gases. Examples of such processes are the removal of water from air, also called heatless drying, or the removal of smaller quantities of impurities from hydrogen. Later this technology was extended to bulk separations such as the recovery of pure hydrogen from a stream containing 30 to 90 mole percent of hydrogen and other readily adsorbable components like carbon monoxide or dioxide, or, for example, the recovery of oxygen from air by selectively adsorbing nitrogen onto molecular sieves.
PSA processes are typically carried out in multi-bed systems as illustrated in U.S. Pat. No. 3,430,418 to Wagner, which describes a system having at least four beds. As is generally known and described in this patent, the PSA process is commonly performed in a cycle of a processing sequence that includes in each bed: (1) higher pressure adsorption with release of product effluent from the product end of the bed; (2) co-current depressurization to intermediate pressure with release of void space gas from the product end thereof; (3) countercurrent depressurization to a lower pressure; (4) purge; and (5) pressurization. The void space gas released during the co-current depressurization step is commonly employed for pressure equilization purposes and to provide purge gas to a bed at its lower desorption pressure.
Similar systems are known which utilize three beds for separations. See, for example, U.S. Pat. No. 3,738,087 to McCombs. The faster the beds perform steps 1 to 5 to complete a cycle, the smaller the beds can be when used to handle a given hourly feed gas flow. If two steps are performed simultaneously, the number of beds can be reduced or the speed of cycling increased; thus, reduced costs are obtainable.
U.S. Pat. No. 4,589,888 to Hiscock, et. al. discloses that reduced cycle times are achieved by an advantageous combination of specific simultaneous processing steps. The gas released upon co-current depressurization from higher adsorption pressure is employed simultaneously for pressure equalization and purge purposes. Co-current depressurization is also performed at an intermediate pressure level, while countercurrent depressurization is simultaneously performed at the opposite end of the bed being depressurized.
The present assignee has developed an effective PSA process for the removal of nitrogen from natural gas streams. The process is described in U.S. Pat. No. 6,197,092, issued Mar. 6, 2001. In general, the process involves a first pressure swing adsorption of the natural gas stream to selectively remove nitrogen and produce a highly concentrated methane product stream. Secondly, the waste gas from the first PSA unit is passed through a PSA process which contains an adsorbent selective for methane so as to produce a highly concentrated nitrogen product. One important feature of the patented invention is the nitrogen selective adsorbent in the first PSA unit. This adsorbent is a crystalline titanium silicate molecular sieve also developed by the present assignee. The adsorbent is based on ETS-4 which is described in commonly assigned U.S. Pat. No. 4,938,939. ETS-4 is a novel molecular sieve formed of octrahedrally coordinated titania chains which are linked by tetrahedral silicon oxide units. The ETS-4 and related materials are, accordingly, quite different from the prior art aluminosilicate zeolites which are formed from tetrahedrally coordinated aluminum oxide and silicon oxide units. A nitrogen selective adsorbent useful in the process described in U.S. Pat. No. 6,197,092 is an ETS-4 which has been exchanged with heavier alkaline earth cations, in particular, barium. It has also been found by the present assignee that in appropriate cation forms, the pores of ETS-4 can be made to systematically shrink from slightly larger than 4 xc3x85 to less than 3 xc3x85 during calcinations, while maintaining substantial sample crystallinity. These pores may be frozen at any intermediate size by ceasing thermal treatment at the appropriate point and returning to ambient temperatures. These materials having controlled pore sizes are referred to as CTS-1 (contracted titano silicate-1) and are described in commonly assigned U.S. Pat. No. 6,068,682, issued May 30, 2000, incorporated herein by reference in its entirety. The CTS-1 molecular sieve is particularly effective in separating nitrogen and acid gases selectively from methane as the pores of the CTS-1 molecular sieve can be shrunk to a size to effectively adsorb the smaller nitrogen and carbon dioxide and exclude the larger methane molecule. The barium-exchanged ETS-4 for use in the separation of nitrogen from a mixture of the same with methane is described in commonly assigned U.S. Pat. No. 5,989,316, issued Nov. 23, 1999. Reference is also made to U.S. Pat. No. 6,315,817 issued Nov. 13, 2001, which also describes a pressure swing adsorption process for removal of nitrogen from a mixture of same with methane and the use of the Ba ETS-4 and CTS-1 molecular sieves. Due to the ability of the ETS-4 compositions, including the CTS-1 molecular sieves for separating gases based on molecular size, these molecular sieves have been referred to as Molecular Gate(copyright) sieves.
An apparent disadvantage of using Molecular Gate(copyright) titanium silicate sieves in processes for the removal of nitrogen from natural gas is that approximately one-half of the propane and all the butane and heavier hydrocarbon components are co-adsorbed with the nitrogen. Thus, it has been found that the C3+ hydrocarbons, although too large to be adsorbed in the pores of the Molecular Gate(copyright) sieves, are adsorbed on the exterior surfaces of the sieves and binder used to hold the sieves together to form a particle. On regeneration of the sieves during the PSA process, the nitrogen and C3+ components are combined as a low pressure tail gas. The C3+ components represent a loss of desirable heating value and additional chemical value when present in the tail gas.
Commonly assigned, co-pending application, U.S. Ser. No. 09/945,870, filed Sep. 4, 2001 is directed to an improved PSA process for removing CO2 from natural gas streams. In general, the process involves an initial PSA separation with a carbon dioxide-selective adsorbent, the formation of an intermediate pressure vent stream such as methane and recycling of the vent stream to feed. CO2-selective adsorbents include activated carbon, alumina, silica, and zeolite molecular sieves. A preferred CO2-selective adsorbent is a silica gel marketed under the name PCS(trademark) by Engelhard Corporation, Iselin, New Jersey. Unfortunately, similar to loss of hydrocarbons found with nitrogen removal using CTS-1 adsorbents, on regeneration of the CO2 adsorbent, it has been found that CO2+ hydrocarbons are combined with the carbon dioxide in the tail gas. Again, the C2+ components in the tail gas represent a loss of desirable heating value and additional chemical value.
The majority of the market supply of C2 and C3+ hydrocarbons are extracted from natural gas. For this reason these components are commonly termed natural gas liquids (NGLs). The removal of the C3+ hydrocarbons from natural gas is accomplished in three alternative routes.
In the first and oldest method, heavy oil is contacted with natural gas such that the lean oil wash absorbs C3+ components into the liquid. These components are then stripped from the oil and eventually recovered as a separate product. More recent designs use refrigerated oil but overall this technology is considered outdated. A second method of recovery of C3+ hydrocarbons is through a refrigeration system where the natural gas feed is chilled to temperatures typically in the range of xe2x88x9230xc2x0 F. and the C3+ components are substantially condensed from the natural gas stream. A more efficient, though more expensive, method and means to recover ethane as well, is generally applied to large gas flows where a turbo-expander plant expands the natural gas to a lower pressure. This expansion causes a substantial drop in the temperature of the natural gas stream. Once more, C3+ hydrocarbons are removed. As a general rule turbo-expander plants are favored where ethane recovery is desired or higher levels of C3+ liquids recovery is justified. These plants are expensive, especially for recompression. All of the routes for liquid recovery are fairly expensive in capital and require considerable power for either refrigeration or recompression.
The relationship in value of natural gas to natural gas liquids is complex and the prices, while related, do fluctuate. Almost always, the components are more valuable as a liquid than as a gas and a typical increase in value is about 1.5xc3x97 the value in the pipeline. The extraction of liquids is the main business of mid-stream processors.
The present assignee has developed processes for the removal of nitrogen and recovery of hydrocarbons from natural gas utilizing pressure swing adsorption with Molecular Gate(copyright) sieves. These processes are described in co-pending applications, U.S. Ser. Nos. 09/699,664, filed Oct. 30, 2000, now U.S. Pat. No. 6,444,012, issued Sep. 3, 2002, and Ser. No. 09/793,039, filed Feb. 26, 2001, now U.S. Pat. No. 6,497,750, issued Dec. 24, 2002. In the former application, the PSA process involves initially adsorbing C3+ hydrocarbons from a natural.gas stream in a first PSA unit containing a hydrocarbon-selective adsorbent to produce a first product stream comprising methane, nitrogen and reduced level of hydrocarbons relative to the feed. The first product stream is then directed to a second PSA adsorption unit containing a nitrogen selective adsorbent (Molecular Gate(copyright)) so as to adsorb nitrogen and produce a second product stream enriched with methane. Recovery of the hydrocarbons can be achieved by desorbing the first adsorbent with the methane product stream. In this way, the heat value of the C3+ hydrocarbons is recaptured in the methane stream. The latter application is directed to a process of separating nitrogen from a feed natural gas stream in a first PSA unit containing a Molecular Gate(copyright) nitrogen-selective adsorbent to form a methane product stream, directing the tail gas from the first PSA unit to a second PSA unit containing a methane selective adsorbent so as to recover methane from the tail gas to form a nitrogen rich product stream and a tail gas stream comprising hydrocarbons and refrigerating the hydrocarbon-containing tail gas so as to knock out the C3+ hydrocarbon liquids. The methane is then recycled to feed.
The process of the present invention which is described below, provides for both the effective removal of nitrogen and/or carbon dioxide from natural gas such as with a Molecular Gate(copyright) sieve and recovery of the C3+ hydrocarbons which are also contained in the natural gas stream. The process of the present invention provides an alternative to previous processes for natural gas liquid recovery from natural gas streams as well as an alternative from the present assignee""s own combined processes of nitrogen removal and hydrocarbon recovery from natural gas streams using pressure swing adsorption with Molecular Gate(copyright) sieves.
This invention provides a novel PSA system to remove nitrogen and/or carbon dioxide from natural gas. The PSA process of this invention to remove nitrogen/CO2 from natural gas also achieves high system NGL recovery. In accordance with this invention, a natural gas feed is first passed through a nitrogen-selective adsorbent or CO2-selective adsorbent, such as a Molecular Gate(copyright) titanium silicate adsorbent of the present assignee, to selectively remove nitrogen and/or CO2 from the natural gas stream and produce a product rich in methane gas. Along with the adsorbed nitrogen and/or carbon dioxide, a significant portion of the C3+ hydrocarbons are adsorbed on the exterior surface of the adsorbent. The C3+ hydrocarbon recovery is achieved by directing the tail gas from the first PSA unit and which is concentrated in desorbed nitrogen and/or CO2 and C3+ hydrocarbons to a second Partial Pressure Swing /Pressure Swing adsorber unit which is selective for the hydrocarbons such as a carbon adsorbent. Note we refer to this unit as a partial pressure swing adsorber because the stream entering the bed on the purge step is not generated by the unit but is an external stream. The natural gas liquids are recovered from the adsorbent of the second PSA unit by desorption with a co-current intermediate pressure vent stream from the first PSA unit.
Subsequent to the adsorption of the nitrogen and/or CO2 in the first PSA unit, one or more pressure equalization steps (depressurizing co-current to the feed) are conducted in which the methane is removed from the adsorber vessel in the step following adsorption and transferred into one or more other vessels undergoing purge or repressurization steps. Such pressure equalization and purge steps in a PSA process are well understood by those of ordinary skill in the art. In traditional PSA processing, at the end of such co-current depressurization steps, the adsorber vessel is depressurized in a direction counter-current to the feed stream and the impurity, in the case of the present invention, nitrogen and/or carbon dioxide, is partially removed. The removal of the impurity is further conducted by purging the bed, typically with a light gas component. In the present invention, rather than following the traditional co-current depressurization steps of equalization or provide purge with a counter-current blow down step, a step or steps of co-current depressurization is used in which the co-current depressurization stream substantially containing the desirable methane is removed at intermediate pressure and directed to the adsorbent in the second PSA unit which contains adsorbed C3+ hydrocarbons. This co-current vent stream at intermediate pressure is able to desorb the NGL components from the adsorbent. The methane and other heavier hydrocarbons can then be separated by flash separation or refrigeration.
The co-current vent stream to the second PSA unit in the process of this invention allows the PSA system to recover natural gas liquids that would otherwise be lost in the tail gas stream of the first impurity-selective PSA unit and further allows the PSA system to further treat methane gas that would otherwise be lost during the blow down step. Accordingly, not only is NGL recovery provided, but overall methane recovery is increased. At the end of the co-current depressurization step, the traditional blow down followed by purge steps and subsequent re-pressurization can be conducted. It may also be desirable to conduct additional co-current depressurization steps such as equalizations after the co-current depressurization vent step.